Phosphonitrilic chloride polymers

ABSTRACT

Linear phosphonitrile chloride polymers are produced from linear phosphonitrilic chloride oligomers by means of a two-step proess. In the first step a mixture of linear phosphonitrilic chloride oligomer having an average degree of polymerization of at least 3 and preferably at least 4 is heated with an excess of ammonia or ammonium chloride having a relatively small particle size such that the Mean Value is within the range of from about 1 micron to about 110 microns. Cyclic phosphonitrilic chloride oligomer formed during the course of the first step is removed from the reaction mixture. After the removal of the cyclic oligomer the reaction mixture is subjected to the second step which involves heating the mixture in an inert liquid solvent (optionally in the presence of ammonia or ammonium chloride having a relatively small particle size) whereby the molecular weight of the linear phosphonitrilic chloride polymer is increased.

REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of my copending application, Ser. No.314,522, filed Oct. 26, 1981, now U.S. Pat. No. 4,374,815.

TECHNICAL FIELD

This invention relates to a process for producing linear phosphonitrilicchloride polymers. More particularly this invention relates to a processin which such polymers are produced from linear phosphonitrilic chlorideoligomers of lower molecular weight.

BACKGROUND

The customary method for the preparation of linear phosphonitrilicchloride polymers involves ring-opening polymerization ofphosphonitrilic chloride trimer. Although workable, this method suffersfrom the fact that for satisfactory results to be achieved, highly purecyclic phosphonitrilic chloride trimer must be used as the monomer. Suchmaterial is difficult and expensive to prepare.

Heretofore some work has been devoted to forming phosphonitrilicchloride polymers from lower molecular weight phosphonitrilic chlorideoligomers. For example in J. Chem. Soc. 1960, 2542-7, Lund et al reportan experiment in which a linear phosphonitrilic chloride oligomer of theformula (PNCl₂)₁₁ PCl₄.2 was heated with ammonium chloride insym-tetrachloroethane under reflux. Polymerization occurred after 5.5hours, at which time the amount of hydrogen chloride evolvedcorresponded to the composition (PNCl₂)₁₀.6 PCl₅. The rubbery productwas extracted with light petroleum giving a significant quantity of adark oil containing 10.5 percent PNCl₂ trimer, the remainder of the oilconsisting of cyclic polymers higher than the heptamer.

Moran in J. Inorg. Nucl. Chem. 30. 1405-13 (1968) investigated thethermal polymerization of the linear compound [Cl(PCl₂ =N)₃ PCl₃ ]PCl₆in evacuated sealed tubes at 300° C. for 5 hours and at 350° C. for 5hours. The phosphorus NMR spectrum of both samples indicated thatpolymers of other chain lengths were formed. The results in the 300° C.case suggested to Moran that polymerization to the longer chain lengthcompound [Cl(PCl₂ =N)₆ PCl₃ ]PCl₆ probably occurred. The NMR spectrum ofthe sample heated at 350° C. indicated to Moran that polymers of bothlonger and shorter chain lengths were formed.

G. Allen et al in Polymer 11, 31-43 (1970) report attempts to preparelinear PNCl₂ polymer by reacting PCl₅ with ammonium chloride inortho-dichlorobenzene, the ammonium chloride being introduced bystepwise addition to the reaction mixture. They were in hopes that thefollowing reactions would occur:

(a) PCl₅ +NH₄ Cl→(1/n) Cl-(PCl₂ =N)_(n) -PCl₄ +4HCl

(b) Cl-(PCl₂ =N)_(n) -PCl₄ +NH₄ Cl→Cl-(PCl₂ =N)_(n) -PCl₂ =NH+3HCl

(c) Cl-(PCl₂ =N)_(n) -PCl₂ =NH+Cl-(PCl₂ =N)_(N) -PCl₄ →Cl-(PCl₂ =N)_(2n)-PCl₄ +HCl

However they obtained very low molecular weight polymer (intrinsic)viscosity of trifluoroethoxy derivative was below 0.05 deciliters pergram). When they attempted to increase the molecular weight of theirpolymer product by reacting it with ammonium chloride inortho-dichlorobenzene solvent, they obtained a crosslinked material.

U.S. Pat. No. 3,443,913 discloses a method wherein linear (PNCl₂)₃₋₁₅oligomers are heated at 240°-260° C. to produce linear phosphonitrilicchloride polymers having a molecular weight between 3,000 and 10,000.However, this process involves heating for long periods of time, theendpoint of the polymerization occurring about 40 to 60 hours afterheating has been initiated. The product obtained via this process isreported to be a dark orange viscous oil. See also James M. Maselli,Thomas Bieniek and Rip G. Rice (W. R. Grace and Company),Phosphonitrilic Laminating Resins, Air Force Materials Laboratory,Technical Report AFML-65-314; Wright-Patterson Air Force Base, Ohio:June, 1965, pages 18-19, which describes this same process. At page 47of this report Maselli et al describe an experiment wherein oligomericphosphonitrilic chloride was placed in a resin kettle fitted with anitrogen inlet, stirrer and exhaust tube condenser. The kettle washeated to 250°±10° C. for a total of 55 hours while the polymeric(PNCl₂)_(n) was stirred under a blanket of dry nitrogen. Samples of thereaction material were taken at selected intervals of time during theheating for molecular weight determination. The resulting data were asfollows:

    ______________________________________                                        Time (Hours) Molecular Weight (VPO)                                           ______________________________________                                        Start        700                                                              10           1200                                                             40           3200                                                             55           6900                                                             ______________________________________                                    

According to the authors, when heating was continued for an additional 8hours at temperatures in excess of 250° C., the viscous, soluble oil(molecular weight 6900) was converted to the familiar insoluble"inorganic rubber".

In U.S. Pat. No. 3,545,942 which in part discloses a method of thermallystabilizing phosphonitrilic chloride oligomers by heating them in aninert atmosphere for 2 to 8 hours at 240° to 260° C., Rip G. Rice et alindicate that prolonged heating of the oligomer can result in theformation of an "inorganic rubber". A decade earlier Lund et al (op.cit.) referred to an experiment in which heating of a linearphosphonitrilic chloride oligomer in tetrachloroethane solution resultedin polymerization after 29 hours.

In prior copending applications Ser. Nos. 956,227 filed Oct. 30, 1978and 176,926 filed Aug. 11, 1980, a distinctly superior thermalpolymerization process is described wherein linear phosphonitrilicchloride oligomer is heated to 275° to 350° C. for 1 to 20 hours whileconcurrently withdrawing phosphorus pentachloride vapor from the liquidphase. A similar procedure is described in Japanese Laid-OpenApplication (Kokai) 55-27,344 published Feb. 27, 1980. In this case alinear phosphazene oligomer usually having a degree of polymerization of3 to 15 is heated under reduced pressure (usually less than 20 mm Hg) toproduce linear polymers. Heating for five hours or more at 100°-300° C.is suggested. Unfortunately, phosphorus pentachloride vapor is extremelycorrosive at elevated temperatures--it tends to rapidly corrode even themost expensive corrosion-resistant metals used in the manufacture ofcorrosion-resistant chemical reactors.

Japanese Kokai 55-56,130 published Apr. 24, 1980 described a method forproducing phosphazene polymers in which a linear phosphazene oligomer isheated in the presence or absence of a solvent at 50° to 300° C. using aLewis base such as urea, thiourea, polyurea or polythiourea as acatalyst for increasing molecular weight.

Japanese Kokai 55-56,129 published Apr. 24, 1980 discloses a process inwhich ammonium chloride is used as the catalyst in a reaction involvingheating phosphazene oligomer at 150°-350° C. in a closed system. Forexample, a solution of linear and cyclic phosphonitrilic chlorideoligomers in dichlorobenzene containing a small amount of ammoniumchloride catalyst was heated at 255° C. for 10 hours in a sealed tube toform the polymer.

Japanese Kokai 55-25,475 published Jan. 23, 1980 describes formation ofphosphazene polymers by reacting a phosphorus source (e.g., P+Cl₂ ; PCl₃+Cl₂ ; PCl₅) with a nitrogen source (e.g., NH₃ ; NH₄ Cl) in any of threereaction systems:

(1) In a solvent that does not dissolve the phosphazene polymers, suchas an aliphatic hydrocarbon or alicyclic hydrocarbon that is resistantto halogenation.

(2) In an undiluted (concentrated) reaction system having a smallquantity (250 ml or less per mole of P source reactant) of a solventcapable of dissolving the phosphazene polymers that is resistant tohalogenation, such as a halogenated aromatic hydrocarbon.

(3) In a phosphazene oligomer as the solvent.

Japanese Kokai 55-65,228 published May 16, 1980 describes a method forproducing phosphazene polymers in which a mixture of linear phosphazeneoligomer, which has been stabilized with phosphorus pentachloride,hydrogen chloride or a metal halide, and cyclic phosphazene oligomer, isheated at 150° to 350° C. in a closed system having a solvent ornon-solvent in the presence of a Lewis base catalyst. Urea, thiourea,polyurea, and polythiourea are examples of Lewis base cataysts used.

Japanese Kokai 55-50,027 published Apr. 11, 1980 discloses performingthermal ring-opening polymerization of cyclic phosphazene oligomers inthe presence of linear phosphazenes stabilized with a metal halide,notably the linear oligomers formed as by-products when synthesizing thecyclic oligomers with metals or metal salts as catalysts. Such linearoligomers are indicated to have a degree of polymerization in the rangeof 2 to 100.

Japanese Kokai 55-60,528 published May 7, 1980 discloses a processwherein phosphazene polymers are formed by heating phosphazene oligomerat 150° to 350° C. in a closed system in the presence of a Lewis basesuch as urea, thiourea, polyurea or polythiourea. The phosphazeneoligomer is a mixture of linear phosphazene oligomers (5 to 95 weightpercent; stabilized with phosphorus pentahalide or hydrogen halide) andcyclic phosphazene oligomer.

Japanese Kokai 55-43,174 published Mar. 26, 1980 describes a process forproducing phosphazene polymers in which cyclic phosphazene oligomers aresubjected to thermal ring-opening polymerization in the presence oflinear phosphazenes which have been stabilized by phosphoruspentahalides or hydrogen halides.

Despite the variety of approaches studied, no completely satisfactorymethod for producing linear phosphonitrilic chloride polymers fromlinear phosphonitrilic chloride oligomers has been reported to date.Among the unsolved problems or shortcomings plaguing the prior methodsnoted above are the following:

formation of polymers of molecular weight lower than desired

formation of impure or cross-linked polymers having undesired propertiesor characteristics

requirement for long reaction or polymerization periods with consequentlow reactor productivity

formation of highly corrosive coproducts such as phosphoruspentachloride at extremely high temperatures which necessitates use ofvery expensive corrosion-resistant reactors

necessity of solvent extraction operations to remove cyclic oligomersby-products and other time-consuming, difficult and costly separationprocedures and their attendant problems

formation of the desired polymer in yields lower than desired

need for very high reaction or polymerization temperatures.

A welcome contribution to the art would be the provision of a processavoiding these difficulties and shortcomings.

THE INVENTION

In accordance with this invention a two-stage process is providedwhereby linear phosphonitrilic chloride polymers can be readily producedfrom linear phosphonitrilic chloride oligomers of lower molecularweight. The problems, difficulties and shortcomings of prior proceduresnoted above are eliminated or at least significantly reduced. Polymersof desired molecular weights (e.g., average degrees of polymerization inthe range of 20 to 1000 or more) can be formed in good yield and highpurity at relatively moderate temperatures in relatively short reactionperiods. The highly corrosive phosphorus pentachloride is not formed andthe process is capable of being performed in relatively simple andeconomical reaction equipment. Complex separation procedures are notrequired.

To achieve these and other attendant technical and economicdisadvantages of this invention, use is made of a process whichcomprises

(a) heating a mixture of linear phosphonitrilic chloride oligomer and anexcess of ammonia or ammonium chloride at a temperature in the range ofabout 130° to about 250° C. while concurrently removing hydrogenchloride and concurrently or subsequently removing cyclicphosphonitrilic chloride oligomer from the heated reaction mixture; and

(b) after formation of cyclic phosphonitrilic chloride oligomer hasessentially ceased and after removal of the oligomer, heating thereaction mixture in an inert liquid solvent, optionally in the presenceof ammonia or ammonium chloride, at a temperature in the range of about130° to about 250° C. to increase the molecular weight of the dissolvedlinear phosphonitrilic chloride polymer.

Preferred conditions for use in the above process involve conductingstep (a) at a temperature in the range of about 140° to about 230° C.and step (b) at a temperature of about 140° to about 240° C. Aparticularly preferred embodiment involves use of temperatures in therange of 150° to 210° C. in (a) and 170° to 240° C. in (b).

An important facet of this invention is to employ in (a) a linearphosphonitrilic chloride oligomer having an average degree ofpolymerization of at least 3 and preferably at least 4. In other words,the oligomer raw material for the process--normally a mixture ofoligomer molecules of somewhat differing molecular weights--should havea number average degree of polymerization of 3 and preferably 4 or more.Thus the oligomer used in (a) may be represented by the formula:

    [Cl-(PCl.sub.2 =N).sub.n -PCl.sub.3 ].sup.+ PCl.sub.6.sup.-

wherein n is a numeral which averages at least 3 and preferably at least4, e.g., a numeral in the range of 3 to 15 or more and preferably in therange of 4 to 15 or more.

In a further preferred embodiment the linear oligomer used in (a)contains less than 35 mol percent of linear oligomer molecules having adegree of polymerization of 3 or less. By employing oligomers meetingthis criterion the amount of competing side reactions such as cyclicoligomer formation is minimized.

Methods for the preparation of linear oligomers as above described arewell known and reported in the literature. One typical procedureinvolves refluxing a mixture of phosphorus pentachloride and ammoniumchloride in a suitable solvent for a suitably long reaction period,distilling off the solvent under reduced pressure and then extractingthe oligomeric product with a suitable extraction solvent such as hexaneto remove residual amounts of the reaction solvent and cyclic oligomersformed during the reaction. One such procedure successfully used inpractice involves forming a mixture of about 6.0 kg of phosphoruspentachloride, about 1.2 kg of ammonium chloride and 3 liters ofmonochlorobenzene. Such mixture is refluxed in a 12-liter reactor atatmospheric pressure for 4 days at about 131° C. Hydrogen chlorideevolved during the reaction is absorbed in aqueous scrubbers. At the endof this time the chlorobenzene solvent is stripped off under reducedpressure at 60° to 80° C. Then the product is extracted with hexane inorder to remove residual amounts of chlorobenzene solvent and cyclicoligomers. Such a product is then ready for use in practicing theprocess of this invention. Another very useful method for preparing thelinear oligomers is described in U.S. Pat. No. 4,198,381. the disclosureof which is incorporated herein.

As noted above the nitrogen source as used in step (a) is preferablyammonia or ammonium chloride, or both. The quantities in which thenitrogen source material is used are not critical provided that asufficient amount is introduced into the reaction mixture to provide anexcess over the stoichiometric amount relative to the quantity of linearoligomer being reacted. Preferably, the entire amount of ammonia orammonium chloride being used in the reaction is charged into thereaction vessel at the start of the reaction. However, the ammonia orammonium chloride can be introduced into the reaction mixture on anincremental or continuous basis during the course of all or a portion ofthe reaction of step (a).

On heating the nitrogen source material with the linear phosphonitrilicchloride oligomer polymerization takes place along with the formation ofcyclic phosphonitrilic chloride oligomer. An important aspect of thisinvention is the removal of this cyclic oligomer from the reactionmixture before performing the further polymerization reaction of step(b) giving rise to the formation of the higher molecular weight linearphosphonitrilic chloride polymer. To effect this removal of cyclicoligomer in (a) use may be made of several different process techniques.For example, all or a portion of (a) may be performed in a boiling inertorganic liquid whereby the liquid vapors drive off the cyclicphosphonitrilic chloride oligomer. Another method is to perform all or aportion of (a) at a reduced pressure so that at the temperature employedthe cyclic phosphonitrilic chloride oligomer is distilled from thereaction mixture. Still another way of effecting the removal of thecyclic phosphonitrilic chloride oligomer is to sweep the heated reactionmixture of (a) with an inert vapor or gas. In this way the entrainedcyclic oligomer is carried away from the reaction zone. In all cases itis desirable that the reaction mixture be suitably agitated both duringthe reaction and during the removal of the cyclic oligomer.

Step (a) may be thus performed in the presence or absence of an inertorganic liquid or diluent. In most cases it is preferable to conduct (a)in bulk as this reduces the size requirements for the reaction vessels.

When employing solvents in step (a) use may be made of such materials assaturated aliphatic hydrocarbons (e.g., nonanes, decanes, dodecanes,etc.), saturated cycloaliphatic hydrocarbons (e.g.,o-dimethylcyclohexane, etc.), aromatic hydrocarbons (e.g, xylenes,trimethylbenzenes, ethylbenzene, methylnaphthalenes, etc.), chlorinatedhydrocarbons (e.g., 1,4-dichlorobutane, tetrachloroethane,chlorobenzene, dichlorobenzenes, etc.), and other similar inertmaterials.

Reaction times used in step (a) should be long enough for the formationof cyclic phosphonitrilic chloride oligomer to essentially cease at thereaction temperature employed. In general reaction times in (a) rangingfrom about 4 to about 12 hours will usually be sufficient.

It has been discovered that the reaction time in (a) can besignificantly reduced by reducing the particle size of the ammoniumchloride used. Thus, it has been determined that conversion of linearoligomer to linear polymer using ammonium chloride having a relativelysmall particle size (see Example VI) was complete after about one hour.On the other hand, when ammonium chloride having a relatively largeparticle size (see Example VII) was used, the polymerization rate wasslower and the yield of desired product was lower--i.e., after about twohours about 40% to about 45% (as determined from NMR spectra) of thelinear oligomer was converted to polymer.

It can therefore be seen from comparing Examples VI and VII that byusing ammonium chloride having a relatively small particle size thereaction rate is superior than when a commercial grade having arelatively large particle size is used.

In general, the particle size of the ammonium chloride should be withinthe range of about 0.1 microns to about 210 microns, such that the MeanValue is within the range of about 1 micron to about 110 microns,preferably within the range of about 1 micron to about 100 microns andmost preferably within the range of about 1 micron to about 90 microns.

In accordance with other preferred embodiments of this invention theammonium chloride is further characterized by satisfying additionalparticle size parameters. Such parameters may be represented by thefollowing designations:

PH, which stands for 10 volume % of particles greater than the value ofthe microns stated

PM, which stands for 50 volume % of particles greater than the value ofthe microns stated

PS, which stands for 90 volume % of particles greater than the value ofthe microns stated

For example, a PH of 145 microns, a PM of 83.8 microns and a PS of 37.4microns means that the sample contains 10 volume % of particles greaterthan 145 microns, 50 volume % greater than 83.8 microns and 90 volume %greater than 37.4 microns, respectively.

Thus in accordance with these further preferred embodiments the ammoniumchloride employed has in addition to the foregoing Mean Values a PHbelow about 180 microns and most preferably below about 160 microns, aPM below about 90 microns and most preferably below about 85 microns,and a PS below about 45 microns and most preferably below about 40microns.

Ammonium chloride having a relatively small particle size may beprepared, for example, by reacting hydrogen chloride gas with ammoniagas as shown in Example VI. It is known that if the ammonium chloride isformed and used in situ without first isolating the ammonium chloride,the particle size will have a Mean Value less than about 86microns--i.e., a Mean Value as low as about 5 microns.

Once the formation of cyclic phosphonitrilic chloride has essentiallyceased and after the cyclic oligomer has been removed from the reactionmixture formed in (a), step (b) is then performed. This involves heatingthe resultant reaction mixture in an inert liquid solvent at a suitablyelevated temperature, optionally in the presence of ammonia or ammoniumchloride, for a time period sufficient to increase the molecular weightof the linear phosphonitrilic chloride polymer. When ammonium chlorideis used, it is preferable to use an ammonium chloride having arelatively small particle size as discussed above for step (a).Generally speaking, the longer the reaction time the higher themolecular weight of the resultant linear phosphonitrilic chloridepolymer. Accordingly, the reaction time for step (b) may be variedwithin relatively wide limits although ordinarily times in the range of4 to about 24 hours will usually be used. As noted, at least a portionof step (b) may be performed in the presence of ammonia or ammoniumchloride (or both) and such material(s) may be introduced into thereaction mixture at the start and/or during the course of step (b).Alternatively, such material(s) may constitute residual ammonia orammonium chloride remaining in the reaction mixture after completion ofstep (a).

If step (b) is performed in the presence of ammonium chloride, it willbe advantageous to use ammonium chloride having a relatively smallparticle size as discussed above for step (a).

A wide variety of inert solvents may be employed in step (b). Theseinclude inert chloroaliphatic, cycloaliphatic, and aromatic solvents ofvarious types. While various cycloalkanes, chloroalkanes andchlorocycloalkanes having appropriate boiling points are thus suitablefor this process, it is preferred to use an inert aromatic solvent suchas aromatic hydrocarbons and chloroaromatic hydrocarbons having boilingpoints at least as high as the reaction temperature being used in (b).Preferred solvents of this type include xylenes, methylnaphthalenes,chlorobenzene, dichlorobenzenes, trichlorobenzenes, etc.

If desired, steps (a) and (b) may both be performed in the same solvent.

The amount of solvent used in (b) is preferably regulated so as to keepthe reaction mixture in a concentrated solution while avoiding excessivegelation. Thus it is desirable to perform step (b) in a relativelyconcentrated reaction solution with periodic or continuous addition ofsolvent to maintain the reaction mixture in a fluid state as thereaction proceeds.

If it is desired to recover the linear phosphonitrilic chloridepolymeric product from the reaction solvent used in (b), varioustechniques are availale for use. For example, the solvent may bedistilled off using an appropriate combination of reduced pressure anddistillation temperature. Alternatively the linear phosphonotrilicchloride polymer may be precipitated from the solvent by the addition ofthe solution to a suitable non-solvent such as pentane or hexane. Theseand other similar techniques will be evident to those skilled in theart.

When it is desired to chemically convert the linear phosphonitrilicchloride polymer into another type of phosphazene polymer, subsequentreactions with an appropriate reactant may be effected in the samereaction solvent as used in step (b). Indeed in such cases it isunnecessary to isolate or recover the linear phosphonitrilic chloridepolymer formed in step (b) as the ensuing reaction(s) may be effected inthe same solution.

Steps (a) and (b) may be conducted in separate reactors. A feature ofthis invention, however, is the fact that both steps may be performed inthe same reaction vessel, provided of course that it is appropriatelysized to handle the quantities of material involved in each step. Thusin accordance with a preferred embodiment of this invention, (a) and (b)are conducted in the same reactor. It is further preferred to introducethe solvent of (b) into such reactor upon the completion of (a).

Additional preferred embodiments of this invention involve thefollowing:

1. Employing ammonia as the nitrogen source in (a) and after theformation of cyclic phosphonitrilic chloride oligomer in (a) hasessentially ceased, sweeping the heated reaction mixture of (a) with aninert vapor or gas so as to remove cyclic phosphonitrilic chlorideoligomer vapor from the reaction mixture.

2. Employing ammonium chloride as the nitrogen source in (a) andremoving cyclic phosphonitrilic chloride oligomer as a vapor eitherduring or after its formation in (a) by sweeping the heated reactionmixture of (a) with an inert vapor or gas.

3. Employing ammonia as the nitrogen source in (a) and after theformation of cyclic phosphonitrilic chloride oligomer of (a) hasessentially ceased, removing cyclic phosphonitrilic chloride oligomervapor from the heated reaction mixture of (a) under reduced pressure.

4. Employing ammonium chloride as the nitrogen source in (a) andremoving cyclic phosphonitrilic chloride oligomer as a vapor eitherduring or after its formation in (a) by heating the reaction mixture of(a) under reduced pressure.

It is particularly preferred to conduct each of the above fouroperations in bulk (i.e., in the absence of any added reaction solvent).

The practice and advantages of this invention will be still furtherapparent from the following illustrative examples which are not to beconstrued in a limiting sense.

EXAMPLE I

Step (a): To a one liter, five-neck round bottom flask, cradled in aheating mantle and fitted with a mechanical stirrer, a thermometer, anitrogen inlet tube and a gas/vapor outlet tube were added 250 g of lowmolecular weight linear phosphonitrilic chloride oligomer having anaverage degree of polymerization (n in [Cl-(PCl₂ =N)_(n) -PCl₃ ]⁺ PCl₆⁻) falling in the range of 4 to 6 and 25.6 g of ammonium chloride. (TheNMR spectrum of this oligomer indicated that about 7 percent of theoligomer had a degree of polymerization of 2 with the balance having adegree of polymerization ranging from 3 to 10.) The mixture was heatedat 155°-165° C. for two hours, then the temperature was raised to 200°C. and nitrogen sweeping was started to carry away cyclicphosphonitrilic chloride oligomer formed during the reaction. Thetemperature was kept at 200±3° C. for about 4 hours and then at200°-210° C. for 11/4 hours. During this time several small samples werewithdrawn from the reaction mixture for P³¹ NMR analysis to determinethe residual content of the cyclic oligomers. The reaction wasterminated at the end of the foregoing 11/4 hour period since theanalyses indicated that the removal of the cyclic oligomers wasessentially complete. The reaction yielded 163.3 g of viscous liquidlinear phosphonitrilic chloride polymer. This was dissolved in 138 ml ofmonochlorobenzene (MCB) yielding 240 ml of a polymer solution.

Step (b): To increase the molecular weight of the polymer formed in step(a), 120 ml of the above phosphonitrilic chloride polymer solution, 70ml of MCB and 0.9 g of ammonium chloride were introduced into a flaskequipped as described in step (a) except that the gas/vapor outlet tubewas replaced with a reflux condenser and an HCl outlet adapter andtubing. The mixture was heated at 131°-132° C. for about 41/2 hours.During this time the reaction mixture had become more viscous due to theincrease in molecular weight of the polymer and thus at that time 30 mlof MCB was added to reduce the viscosity of the reaction mixture. Eightyminutes later an additional 30 ml of MCB was added to thin the reactionmixture and the reaction was temporarily discontinued by turning off theheating system and the stirrer. Next morning the reaction was resumed.To accelerate the reaction, an additional 0.1 g of NH₄ Cl was added. Thetemperature of the reaction mixture was raised to 131°-132° C. and keptat this temperature range for 2 hours. The reaction mixture was furtherthinned with 30 ml of MCB. The reaction was allowed to extend for anadditional 30 minutes before being terminated. Since the reactionmixture was still quite viscous, 5 ml of 1-pentanol dissolved in 20 mlof MCB was added to reduce the viscosity of the polymer solution. Thevolume of the polymer solution was 486 ml and the weight of thephosphonitrilic chloride polymer in the solution was approximately 80 g.

To determine the molecular weight of the phosphonitrilic chloridepolymer, a small sample of poly(phenoxy phosphazene) was produced fromthe above phosphonitrilic chloride polymer. To accomplish this, 102 mlof the above polymer solution was diluted with 100 ml of toluene, thediluted solution was allowed to stand over-night to allow a trace amountof unreacted NH₄ Cl to settle, and the clear solution was then reactedwith 0.32 mole of sodium phenoxide in 200 ml of diglyme for 24 hours at110° C. In this operation 24.4 g of poly(phenoxy phosphazene) wasformed. The intrinsic viscosity of the poly(phenoxy phosphazene) asmeasured in tetrahydrofuran (THF) at 25° C. was 0.49 dl/g.

EXAMPLE II

Step (a): The apparatus was similar to that described in Step (a) ofExample I except that the reactor was a 2 liter, five-neck round bottomflask instead of a 1 liter, five-neck round bottom flask.

A 603 g portion of the same low molecular weight linear phosphonitrilicchloride oligomer as described in Example I and 50 g of NH₄ Cl wereintroduced into the reactor. The mixture was heated at 155°-160° C. for22/3 hours, and at 175°-180° C. for 31/4 hours. The nitrogen sweepingwas then started and the temperature was raised to and kept at 200±2° C.for 21/2 hours. An additional 8 g of NH₄ Cl was added and the reactionwas continued for another 11/2 hours at 200°-210° C. Use of P³¹ NMRanalysis indicated that the removal of the cyclic phosphonitrilicchloride oligomer formed in the reaction was essentially complete andaccordingly the reaction was terminated and the contents of the reactorwere allowed to cool to about 100° C. 500 ml of o-dichlorobenzene (DCB)was added to dissolve the phosphonitrilic chloride polymer (434.4 g).The volume of the polymer solution was 730 ml.

Step (b): 135 ml of the above phosphonitrilic chloride polymer solution(containing about 80 g of the polymer), 2 g of NH₄ Cl and 65 ml of DCBwere added into the same reactor used in Step (b) of Example I. Themixture was heated at 170° C. for 5 hours. The reaction medium was veryviscous and began to climb up the shaft of the stirrer. The reaction wasterminated immediately by turning off the heating and stirring devices.When the temperature of the contents had cooled to about 150° C., 10 mlof 1-hexanol mixed with 100 ml of DCB was added. The stirrer was turnedon. In about 20 minutes the very thick honeylike mass had turned intoviscous liquid. To reduce further the viscosity of the liquid, anadditional 180 ml of DB was added. The volume of the resultantphosphonitrilic chloride polymer solution was 490 ml. Approximately 24 gof insoluble gel-like material remained in the reactor. The weight ofthe polymer dissolved in the solvent was approximately 55 g.

Following the same procedure described in Example I, a small sample ofpoly(phenoxy phosphazene) was prepared for intrinsic viscosity andmolecular weight determinations. The intrinsic viscosity of the polymer(THF; 25° C.) was 0.54 dl/g.

EXAMPLE III

This example illustrates the advantage of diluting the reaction mixtureduring the course of step (b) to prevent gelation as occurred in ExampleII.

A separate 135 ml portion of the same phosphonitrilic chloride polymersolution as used in Step (b) of Example II, 2 g of NH₄ Cl and 65 ml ofDCB were heated in the same reactor of Step (b) of Example II at 170° C.for 32/3 hours. Thereupon 100 ml of DCB was added to thin the reactionmedium. The reaction mixture was continuously heated at the sametemperature for an additional 2 5/6 hours and diluted with two portionsof 100 ml of DCB during this period of time. At the end of a grand totalof 61/2 hours of reaction at 170° C. the heating system was turned offand 50 ml of DCB was added to reduce the rather viscous polymersolution. When the temperature was at about 155° C., 6 ml of 1-hexanolmixed with 60 ml of DCB was added to further reduce the viscosity of thepolymer solution. The reaction yielded 634 ml of the polymer solutioncontaining 79 g of phosphonitrilic chloride polymer. No gel materialreamined in the reactor.

A small sample of poly(phenoxy phosphazene) was prepared as above. Theintrinsic viscosity (THF; 25° C.) was found to be 0.60 dl/g.

EXAMPLE IV

In order to obtain an indication of the relative amounts of linearpolymer, cyclic oligomer, and HCl formed in the step (a) procedure, step(a) was conducted while separately trapping the latter two by-productsfor assay. In particular, a 595 g portion of the same low molecularweight linear phosphonitrilic chloride oligomer as described in ExampleI and 50 g of NH₄ Cl were introduced into the same reactor used in Step(a) of Example II. The mixture was heated at 160° C. for 3 hours, and170° C. for one hour. Nitrogen sweeping was then started to remove thecyclic phosphonitrilic chloride oligomer formed from the reaction. Thetemperature was raised to and kept at 180° C. for one hour, at 200°-205°C. for one hour, and finally at 210° C. for 2 hours. The HCl gasgenerated during the reaction was absorbed in water and the HCl aqueoussolution was titrated with standard NaOH solution. The cyclic oligomeras purged from the reaction vessel was trapped in toluene and uponcompletion of the reaction the toluene was evaporated with the cyclicoligomer residue weighed.

The reaction yielded 416.8 g of linear phosphonitrilic chloride polymer,137 g of HCl and 20 g of cyclic phosphonitrilic chloride oligomers.

EXAMPLE V

Step (a): The apparatus was similar to that described in Example Iexcept that the reactor was a 3 liter, five-neck round bottom flaskinstead of a 1 liter, five-neck round bottom flask.

A further 1.349 kg portion of the same low molecular weight linearphosphonitrilic chloride oligomer referred to in Example I and 127 g ofNH₄ Cl were heated in the above reactor at 160° C. under a nitrogensweep for 22/3 hours, at 170°-180° C. for 3 hours, at 190°-200° C. forone hour and 200°-210° C. for one hour and finally at 210°-220° C. for 4hours. 950 ml of DCB was added to dissolve 918 g of phosphonitrilicchloride polymer product. The volume of the polymer solution was 1,445ml.

Step (b): The apparatus was similar to that described in Step (b) ofExample I except that the reactor was a 5 liter, four-neck round bottomflask.

710 ml of the phosphonitrilic chloride polymer solution from Step (a) ofthis example, 900 ml of DCB and 12 g of NH₄ Cl were heated at 170° C.for 61/2 hours. Four 200-250 ml portions of DCB were added stepwiseduring the last 21/2-hour period of the reaction to reduce the viscosityof the reaction medium. When the temperature of the viscous polymersolution was cooled to 135° C., 10 ml of 1-hexanol mixed with 50 ml ofDCB was added to further reduce the viscosity of the polymer solution.

The reaction yielded 445 g of linear phosphonitrilic chloride polymer,and 3.5 g of HCl was captured in an aqueous trap. The concentration ofthe final polymer solution was 0.174 g/ml.

A small sample of poly(phenoxy phosphazene) was then prepared as abovefrom this final phosphonitrilic chloride polymer solution. The intrinsicviscosity (THF; 25° C.) of the phenoxy substituted polymer was 0.61dl/g.

EXAMPLE VI

The ammonium chloride utilized in this example was prepared in thefollowing manner.

Chlorobenzene, as a solvent, was added to a multineck flask. Throughseparate lines, at opposite ends of the flask, hydrogen chloride gas andammonia gas were bled into the flask below the solvent line--i.e., intothe solvent. The reaction medium was stirred rapidly until a thickslurry of ammonium chloride was obtained. The reaction product wasfiltered under nitrogen. The ammonium chloride was transferred to around bottom flask and placed under a vacuum to remove any remainingsolvent.

The particle size of the ammonium chloride was determined using aMicrotrac Particle Size Analyzer, standard range, (Leeds & Northrup Co.,Microtrac Division) with a small volume cell and the ammonium chloridedispersed in toluene. The particle size range of the ammonium chloridewas determined to be 1.9 microns to 125 microns.

The particle size data for the ammonium chloride produced above is givenin Table I.

                  TABLE I                                                         ______________________________________                                        Synthesized Ammonium Chloride Particle Size                                   Run 1            Run 2     Average                                            (Microns)        (Microns) (Microns)                                          ______________________________________                                        MV     88.0          84.2      86.1                                           PH     145.0         141.0     143.0                                          PM     83.8          78.9      81.4                                           PS     37.4          34.1      35.8                                           ______________________________________                                    

100 g. of low molecular weight phosphonitrilic chloride linear oligomersprepared similarly to that of Example I and 10.0 g of ammonium chloride(prepared as above and determined to have the particle size given inTable I) were placed in a 250 ml creased round-bottom flask equippedwith a paddle stirrer, gas inlet and outlet, and a thermometer. Thevapor space of the flask was swept with nitrogen gas at a rate of 3 SCFH(Standard Cubic Feet per Hour). The paddle stirrer was turned at a rateof 300 rpm. The contents of the flask were heated rapidly to 110° C. andthen heated to 160° C. over a period of one hour. A sample was withdrawnat this time. ³¹ P NMR analysis showed that removal of the cyclicoligomer and conversion of low molecular weight linear oligomer toviscous liquid polymer was essentially complete. The polymer product issimilar to that described in step (a) of Example I.

EXAMPLE VII

The particle size of a commercially available ammonium chloride(available from MCB Manufacturing Chemists, INC., 480 Democrat Road,Gibbstown, N.J. 08027) was determined using a Microtrac Particle SizeAnalyzer, extended range, with a small volume cell and the ammoniumchloride dispersed in toluene. The particle size range was determined tobe 3.3 microns to 212 microns.

The particle size data for the MCB ammonium chloride is furnished inTable II.

                  TABLE II                                                        ______________________________________                                        MCB Ammonium Chloride Particle Size                                           Run 1            Run 2     Average                                            (Microns)        (Microns) (Microns)                                          ______________________________________                                        MV     116.0         113.0     114.5                                          PH     207.0         200.0     203.5                                          PM     106.0         103.0     104.5                                          PS     47.1          47.3      47.2                                           ______________________________________                                    

Using a reaction vessel and reaction conditions as described in ExampleVI, another 100 g of the same low molecular weight phosphonitrilicchloride linear oligomer prepared and used in Example VI and 10.0 g ofMCB ammonium chloride were combined and heated rapidly to 110° C. Thetemperature was then raised from 110° C. to 160° C. over a period of onehour. A sample was withdrawn at this time and thereafter an additionalsample was withdrawn every twenty minutes for a total of threeadditional samples while maintaining the reaction temperature at 160° C.³¹ P NMR analysis of the final sample showed that about 40% to about 45%of the low molecular weight linear oligomer was converted to viscousliquid linear polymer.

The linear phosphonitrilic chloride polymers produced in accordance withthis invention are useful for a variety of applications. By way ofexample these linear polymers when of relatively low molecular weightare useful as intermediates in the synthesis of hydraulic fluids,lubricants and flame retardants. In particular the linearphosphonitrilic chloride polymers preferably having average degrees ofpolymerization in below about 50 may be substituted with aryloxy and/oralkoxy groups to form products useful as hydraulic fluids, lubricantsand flame retardants. Methods for effecting such substitution are wellknown in the art and are described for example in U.S. Pat. Nos.3,443,913; 3,856,712; 3,883,451; and 4,055,523. Alternatively aryloxyand alkoxy substituted linear polymers of higher average degrees ofpolymerization containing ethylenic unsaturation can be compounded andcured by cross-linking to produce elastomers, coatings, adhesives,potting compounds, thermoset plastics and flexible or rigid foams. Notein this connection U.S. Pat. No. 4,264,531.

I claim:
 1. A process for producing linear phosphonitrilic chloridepolymers from linear phosphonitrilic chloride oligomers of lowermolecular weight which comprises:(a) heating a mixture of linearphosphonitrilic chloride oligomer having an average degree ofpolymerization of at least 3 and an excess of ammonium chloride at atemperature in the range of about 130° to about 250° C. whileconcurrently removing hydrogen chloride and concurrently or subsequentlyremoving cyclic phosphonitrilic chloride oligomer from the heatedreaction mixture, said ammonium chloride having a particle size withinthe range of about 0.1 microns to about 210 microns such that, the MeanValue is within the range of about 1 micron to about 110 microns; and(b) after the formation of cyclic phosphonitrilic chloride oligomer hasessentially ceased and after removal of the oligomer, heating thereaction mixture in an inert liquid solvent at a temperature in therange of about 130° to about 250° C. to increase the molecular weight ofthe dissolved linear phosphonitrilic chloride polymer.
 2. A processaccording to claim 1 wherein the oligomer has an average degree ofpolymerization of at least
 4. 3. A process according to claim 1 whereinsaid oligomer is further characterized by containing less than 35 molepercent of linear oligomers having a degree of polymerization of 3 orless.
 4. A process according to claim 1 wherein (a) is conducted at atemperature within the range of about 150° to about 210° C. and (b) isconducted at a temperature within the range of about 170° to 240° C. 5.A process according to claim 1 wherein at least a portion of (a) isperformed in bulk.
 6. A process according to claim 1 wherein saidammonium chloride has a Mean Value within the range of from about 1micron to about 100 microns.
 7. A process according to claim 1 whereinsaid ammonium chloride has a Mean Value within the range of from about 1micron to about 90 microns.
 8. A process according to claim 1 whereinduring at least a portion of (a) the ammonium chloride is introducedinto the reaction mixture continuously or incrementally.
 9. A processaccording to claim 1 wherein removal of the cyclic phosphonitrilicchloride oligomer vapor in (a) is effected by sweeping the heatedreaction mixture with an inert vapor or gas.
 10. A process according toclaim 1 wherein removal of the cyclic phosphonitrilic chloride oligomerin (a) is effected by reducing the pressure on the heated reactionmixture to below atmospheric pressure.
 11. A process according to claim1 wherein at least a portion of (a) is performed in a boiling inertsolvent whereby the solvent vapors drive off cyclic phosphonitrilicchloride oligomer.
 12. A process according to claim 1 wherein at least aportion of (b) is performed in the presence of ammonium chloride havinga particle size within the range of about 0.1 microns to about 210microns such that the Mean Value is within the range of about 1 micronto about 110 microns.
 13. A process according to claim 1 wherein atleast a portion of (b) is performed in the presence of ammonium chloridehaving a particle size within the range of about 0.1 microns to about210 microns such that the Mean Value is within the range of about 1micron to about 100 microns.
 14. A process according to claim 1 whereinthe solvent in (b) is an inert aromatic solvent.
 15. A process accordingto claim 14 wherein the solvent in (b) is a chloroaromatic solvent. 16.A process according to claim 15 wherein the solvent in (b) ischlorobenzene or a dichlorobenzene.
 17. A process according to claim 16wherein the solvent in (b) is ortho-dichlorobenzene.
 18. A processaccording to claim 1 wherein the amount of solvent present in (b) isregulated to keep the reaction mixture in a concentrated reactionsolution while avoiding excessive gelation.
 19. A process according toclaim 1 wherein (a) and (b) are conducted in the same reactor.
 20. Aprocess according to claim 19 wherein the solvent of (b) is introducedinto the reactor upon completion of (a).
 21. A process according toclaim 20 wherein the solvent in (b) is an inert aromatic solvent.